Update:
I can now access the level since all spllne- and data-asset splines have been removed. I can work in it without a problem, until I add a spline- and data-asset based PCG: I added one such a PCG, at first it’s OK, but at some point the perspective+lit window turns ‘blank’ again. Saved all, re-opened the level, no change. Switch to 4-window mode, set top (right bottom) to wireframe and that shows. Next select an asset (in the center of the scene) and perspective shows again, I can open that window in whole screen and go on working.
So my suspicion is that this type of PCG causes problems when generated. Even if clear, it may happen.
Update:
Is isn’t needed to switch to four planes and set any of them to “wireframe”. If the perspective screen turns ‘blank’, select and double-click any object in the outliner, and you will get that asset in view and you can do whatever you like.
However, if the viewport is not blank but errraticly changing when you move your view, this doesn’t work; you’ll need to reopen the level after “save all” (reported Viewport becomes unusable after moving cursor - General / Feedback & Requests - Epic Developer Community Forums (unrealengine.com)